To me, "Ecotone" meant two things: 1) Utter bittersweet perfection and 2) the beginning of the end. The first remark is a positive one, as I thought the episode was very cleverly written, it pushed all the right buttons. It was a worthy ending to Nate's storyline, the guy we all loved to bits and were still a little bit mad at. I never quite got over the indignation I felt when Nate left Brenda at the end of season two to shack up with preggers Lisa. This may have tainted my further view of him but I still sympathized. So I thought it was a master-stroke to proceed Nate's demise by him having sex with Maggie, who had always played a rather dubious role on the show. You knew the two were going to hook up right from the first moment they met, but that it would mean his death? Wow, so much for predictability!
The fact that Nate still got to talk to all those who were close to him (except for Ruth - which was in a way fitting) was very well done. The dream sequence with David was utter perfection. My negative thoughts are these: I found this episode to be almost literally "the beginning of the end". The ones that came after were also quite shocking but didn't have that same punch to me. It's like there couldn't be any more feelings of shock or sadness after the departure of Nate. Not because I loved his character that much, but I just sort of "endured" the rest of the season, without it much affecting me. I generally found "Ecotone" the more effective episode of the last four because initial shock tends to hit hard.